This invention relates to a process for the production of hydrogen peroxide by the anthraquinone process and is directed in particular to the use of the oxidation waste gas as an energy carrier in various processing steps of the overall process.
As is generally known, in the so-called anthraquinone process for producing hydrogen peroxide, a working solution, which contains one or more anthraquinone derivatives as reaction carriers dissolved in an organic solvent or mixed solvent, is hydrogenated in the presence of a suspension catalyst or of a fixed-bed catalyst. In this hydrogenation step, at least part of the reaction carrier is converted into the corresponding anthrahydroquinone derivative. In the subsequent oxidation step the hydrogenated working solution, freed from the catalyst, is gassed with oxygen or with an oxygen-containing gas, in most cases air, and the reaction carrier is converted back into the anthraquinone form, with the production of hydrogen peroxide. In the subsequent extraction step, the hydrogen peroxide contained in the oxidized working solution is extracted with water or with a diluted aqueous hydrogen peroxide solution. The hydrogen peroxide can also be recovered from the oxidized working solution by means of a desorption process, instead of an extraction. The recovered working solution is again recirculated to the hydrogenation step.
In addition to the processing steps mentioned above, the anthraquinone process includes a number of other processing steps, which are essential for an economically efficient operation. These include: concentration and purification of the aqueous hydrogen peroxide solution obtained; drying of the working solution recovered from the extraction step prior to its being recirculated to the hydrogenation step; regeneration of the working solution for the purpose of converting into active components those components of the reaction carrier which have become inactive and regeneration of the hydrogenation catalyst for the purpose of reactivating it. A summary of the anthraquinone process, which is relied on and incorporated herein by reference, is found in Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., 1989, Vol. A13, pages 447–457.
The oxidation step is carried out in one or more serially connected oxidation reactors, which are operated in a cocurrent or countercurrent manner; a combination of the two modes of operation is also known. Where the oxidation step is carried out industrially, in particular such measures are taken as will enable the residence time of the working solution in the oxidation step to be kept as short as possible, in order to minimize the extent of secondary product formation. Besides the citation given above, examples of methods of carrying out the oxidation step may also be found in DE-Auslegeschrift 20 03 268 and in the U.S. Pat. Nos. 3,073,680 and 3,752,885.
According to WO 86/06710, the oxidation can be carried out within a short period and with little technical expense by intensively gassing the hydrogenated working solution with an oxidizing gas in a cocurrent reactor at temperatures of below 100° C. and at excess pressures of below 15 bar, with the formation of a coalescence-inhibited system, and by separating the coalescence-inhibited system, after it has passed through the oxidation reactor or reactors, into a liquid phase and an oxidation waste gas.
In the process just considered and in other oxidation processes using air, the oxidation waste gas, before this can be released into the atmosphere, is expanded and then freed from organic constituents of the solvent in a waste-gas purifier. The waste-gas purifiers are in particular adsorption devices, such as adsorption towers, which are filled with a suitable adsorbent material, such as activated carbon or a natural or synthetic oxide or siliceous adsorbent, including zeolites. Activated carbon is the preferred adsorbent material. Alternatively to this, the oxidation waste gas can be purified by means of a liquid absorber (gas washer) or by freezing out the organic constituents.
As the oxidation reactors are operated using air under excess pressure, the oxidation waste gas also leaves the oxidation reactors still at an increased pressure, even in the case of a high conversion of the oxygen in the air. Before the oxidation waste gas is passed to the waste-gas purification step for the purpose of depleting it of contained organic constituents, the waste gas, which has generally been freed beforehand from dissolved constituents in a condensation step, has to be largely or completely expanded. Where activated carbon towers are used for the purpose of waste-gas purification, these are cyclically regenerated by desorbing the adsorbed solvent by means of medium-pressure steam.
As is known from DE 40 29 784 or from the previously cited Ullmann reference (pages 453–454), the oxidation step can be carried out free from waste gas by using pure oxygen instead of air, but this mode of operation is less economically efficient than the use of air. Accordingly, the invention is not directed towards processes involving a waste-gas free oxidation step.
A disadvantage of the known process for carrying out the oxidation step using air with subsequent expansion and purification of the oxidation waste gas is that a considerable quantity of energy is destroyed during the expansion.
Accordingly, an object of the present invention is to demonstrate a way whereby the energy contained in the oxidation waste gas can be used in the overall process for producing hydrogen peroxide and hence to increase the economic efficiency of the process.
A further object is to lower the input of external energy for producing the vacuum required at various points in the anthraquinone process.